60 DISCUSSION 
s P 
Tian £ (mb) 
KWAJALEIN REDWING  } !00 
/ 
ie i — MEAN RAINY 
—-— MEAN DRY 
-707 
- 200 
-60/7 
=507 L 300 
Fuel L 400 
=30// - 500 
eee 
-207 
700 
- 800 
lor 
Ns 7-900 
a amma —— == 1000) 
) 10 20 30 
Tee 
Fic. 12—Average soundings on the rainy and 
dry days 
Taste 1—Rainy versus dry soundings for 
Kwajalein Atoll, April 15-July 24, 1956 
Stand- 
| Aver. ane 
No. of 24-hour| Aver. | precip- Ae 
days rain lapse | itable resi 
water itable 
water 
Inch /°/100 m gm gm 
Rainy days, 7 1.24 | 0.56 | 5.35 | 0.40 
Dry days, 13 0 | 0.58 | 4.99 | 0.44 
through Kwajalein, and although we have not 
yet determined it for April-June, it probably did 
not lie very far to the south of the Marshalls. 
The procedure was first to calculate the 24-hr 
rainfall occurring between each midnight and the 
following, and arrange the days in a descending 
order. 
In a total of 100 days, 87 experienced rain 
(from a trace upward) and a total of 35.21 
inches of rain fell. The maximum rain falling 
in any 24-hr period was 3.26 inches on May 28, 
and a total of ten days (fairly evenly distributed 
throughout the period; only two consecutive) 
experienced more than one inch. On the highest 
seven of these (R = 1.24 inches) the rain fell 
fairly evenly around the clock, and these were 
designated for the remainder of the study as 
‘rainy’ days. The thirteen days on which no rain 
at all fell were designated as ‘dry’ days. A mean 
sounding was then constructed for the rainy and 
dry days separately (shown in Fig. 12) with 
rather striking results. First, the soundings are 
fantastically similar, and day-to-day variations 
within the ‘rainy’ and ‘dry’ classes far exceeded 
the difference between the two averages. This 
clearly demonstrates the utter futility of at- 
tempting to forecast or explain rainfall in this 
area in terms of the stability or moisture struc- 
ture of vertical soundings alone; we must look 
also either to large-scale dynamics or to micro- 
physics, or to both in interaction. Table 1 sum- 
marizes some of the essential features of the 
soundings. 
It will be noted that the tropospheric lapse 
rate (950 mb—tropopause) is five per cent 
steeper on the dry than on the rainy days, in 
good agreement with other findings in the tropics 
of an inverse relation between instability and 
penetrative clouds. The difference in average 
precipitable water (computed both from indi- 
vidual soundings, then averaging, and from the 
average soundings of Fig. 12) is very slight, and 
the standard deviation of this property within 
each category of days (last column) is greater 
than the difference in the averages, which sug- 
gests that the latter is not too meaningful. 
Thus we must look for reasons for the enormous 
difference in rain in the two categories either in 
terms of the dynamics of the large-scale flows 
and their effects on penetrative cloud growth, or 
just possibly in terms of the differences in micro- 
features of clouds (such as drop spectrum, nuclei 
count and spectrum, ete). 
All evidence to date convinces us that the 
dynamics plays the controlling role, and if micro- 
physical differences are at all significant they too 
are controlled by the large-scale dynamics and 
the convection itself by some as yet unknown 
‘feedback’ mechanism. 
In retrospect, it is not surprising that the con- 
centration of rainfall may prove more pro- 
nounced over oceans, since islands and land 
masses, particularly mountainous ones, create 
both some large-scale convergence themselves 
(due to the diurnal heating-cooling cycle) and 
provide critical ‘concentration points’ for con- 
vergence which would locally intensify that of 
a very weak traveling disturbance. 
